1. Field of the Invention
This invention relates to an acoustic lens for use in an acoustic microscope comprising an ultrasonic wave propagating solid state medium having opposite end surfaces, an electric-acoustic piezoelectric transducer applied on one end surface of said solid state medium, and a lens portion formed in the other end surface of said solid state medium.
2. Description of the Prior Art
Measurements utilizing acoustic energy have been applied in various applications such as sonar, defect detector and fish finder technologies. In medical applications, ultrasonic diagnosing apparatus has been widely used. Recently there has been developed an acoustic microscope which utilizes the transmissibility of an ultrasonic wave through a specimen as well as the modulation of the ultrasonic wave due to the elastic characteristics of the specimen. With the aid of such an acoustic microscope it is possible to observe an image of the elastic specimen at a high resolution. The frequency of the ultrasonic wave used in the acoustic microscope is usually set to several hundred megahertz, but recently an acoustic microscope using ultrasonic waves of very high frequency, e.g., up to the order of gigahertz has been developed. For instance, when water is used as the liquid medium between the acoustic lens and the specimen, it is possible to obtain a high resolution of about 1 .mu.m by using an ultrasonic wave of 1 GHz. Such a resolution is comparable with that of usual optical microscopes. If liquid helium or liquid nitrogen is inserted between the acoustic lens and the specimen, there is a possibility that a higher resolution than 1 .mu.m could be attained.
FIG. 1 is a schematic view showing a typical known acoustic microscope. An acoustic lens 1 comprises an ultrasonic wave propagating solid state medium 2 made of material such as sapphire and fused quartz having a high acoustic propagation velocity, an electric-acoustic piezoelectric transducer 3 applied on one end surface of the solid state medium 2, and a lens portion 4 formed in the other end surface of the solid state medium 2. A high frequency pulse generated by a high frequency pulse generator 5 is supplied to the transducer 3 via a circulator 6, and the transducer 3 produces a plane ultrasonic wave. The ultrasonic wave propagates within the solid state medium 2 and is converged into a spherical wave by the spherical lens portion 4. Between the acoustic lens 1 and a specimen 9 is placed an acoustic wave propagating liquid medium 10 such as water, and the converted spherical wave is projected onto the specimen 9 as a microscopic spot via the liquid medium 10. In the acoustic microscope of the reflection type, the ultrasonic wave reflected by the specimen 9 is collected by the lens portion 4, and then is made incident upon the transducer 3 which converts the received ultrasonic wave into an electric signal. The electric signal is then supplied to a signal processing circuit 7 via the circulator 6 and the signal processing circuit produces a video signal. The video signal is then supplied to a monitor 8 to display an ultrasonic image of the specimen 9. When the acoustic lens 1 and specimen 9 are moved two-dimensionally relative to each other to effect the mechanical scan, a two-dimensional image of the specimen due to the elasticity can be displayed.
In the reflection type acoustic microscope, when the acoustic beam is focused onto a surface of the specimen, it is possible to obtain the acoustic image having a construct in accordance with the difference in the reflection factor for the acoustic wave of the specimen surface. When the specimen is brought closer to the acoustic lens, the incident angle of the spherical acoustic wave emanating from the acoustic lens and impinging upon the specimen changes continuously from 0.degree. to an angle formed between the outermost beam and a principal axis of the acoustic wave. Then the acoustic wave reflected by the specimen is modulated by various components in the specimen in different manners, and the reflected acoustic wave has a phase variation specific to the composition of the specimen. Therefore, by effecting the X-Y scan, it is possible to obtain an image having a contrast in accordance with the acoustic property of substances in the specimen. Further, when the acoustic lens is moved in the direction Z normal to the surface of the specimen to effect a linear scan and an output signal from the acoustic lens is plotted versus the distance in the direction Z, it is possible to attain a so-called V(Z) curve which is specific to the specimen. The above mentioned three functions of the acoustic microscope are very important. For instance, from the acoustic image of the specimen surface, it is possible to detect defects in the specimen surface. When the specimen surface is placed closer to the acoustic lens than the focal point, crystal construction and crystal boundary can be detected from the acoustic image. Moreover, from the V(Z) curve, one can specify or identify or more components in the specimen.
Various studies have been done for the acoustic lens for use in the acoustic microscope, and various acoustic lenses and analyses thereof have been disclosed in the following references.
(1) "ACOUSTIC MICROSCOPY BY MECHANICAL SCANNING", by R. A. Lemons, May 1975, Microwave Laboratory, W. W. Hansen Laboratories of Physics, Stanford University Stanford, Calif.
(2) "CHARACTERISTIC MATERIAL SIGNATURES BY ACOUSTIC MICROSCOPE" by R. D. Weglein and R. G. Wilson in "ELECTRONICS LETTERS", Vol. 14, No. 12, June 6, 1978.
(3) "An Angular-spectrum approach to contrast in reflection acoustic microscopy" by Abdallah Atalar in "JOURNAL OF THE APPLIED PHYSICS", Vol. 49. No. 10, pp 5130-5139, October 1978.
(4) "MODULATION TRANSFER FUNCTION FOR THE ACOUSTIC MICROSCOPE" by Abdallah Atalar in "ELECTRONICS LETTERS", Vol. 15, No. 11, May 24, 1979.
(5) "RAY INTERPRETATION OF THE MATERIAL SIGNATURE IN THE ACOUSTIC MICROSCOPE" by W. Parmon and H. L. Berton in "ELECTRONICS LETTERS", Vol. 15, No. 21, Oct. 11, 1979.
(6) Japanese Patent Application Laid-Open Publication (Kokai) No. 58-44,343.
(7) Japanese Patent Application Laid-Open Publication No. 60-149,963, Japanese Patent Publication No. 59-50,937 and Japanese Utility Model Application Laid-Open Publication No. 57-120,250.
In reference (1), there is disclosed an acoustic lens as shown in FIG. 2 of the present application. The acoustic lens comprises a sapphire rod (Al.sub.2 O.sub.3) 11, an Au electrode 12 applied on one end surface of the rod, a piezoelectric film 13 (ZnO) applied on the Au electrode 12, and an Al electrode 14 applied on the ZnO film 13. In the other end surface of the rod 11 there is formed a spherical lens portion 15. The dimension of the electric-acoustic transducer is defined by the dimension of the uppermost Al electrode 14. As the acoustic lens for 1 GHz, the following parameters have been proposed:
l=2.00 mm PA1 r=0.135 mm PA1 .theta..sub.max =50.degree. PA1 D=0.207 mm PA1 d=0.156 mm
wherein l is the length of the acoustic wave propagating solid state medium 11, r is the radius of curvature of the spherical lens portion 15, .theta. is the aperture angle, D is the aperture diameter and d is the focal distance. This known acoustic lens has the F/number, defined by d/D, of 0.75. In this acoustic lens, the acoustic energy impinging upon portions outside the aperture of the lens portion 15 becomes useless and might interfere with the acoustic energy passing through the lens portion 15. Therefore, when designing the acoustic lens, the dimension of the transducer, i.e. the diameter of the Al electrode 14, has to be adjusted such that the above-mentioned disturbing acoustic energy is minimized. Further, in order to protect the acoustic lens from damage or breakdown, the above dimension must be determined such that the acoustic energy is spread as widely as possible. In order to satisfy such requirements, it has been recommended that the diameter of the Al electrode 14 be made substantially equal to the aperture diameter D of the lens portion 15 and the length l of the medium 11 be selected such that the lens aperture is situated just in a Fresnel focal point or slightly longer than that. Here, the Fresnel focal distance l.sub.0 is given by l.sub.0 =.rho..sub.0.sup.2 /.lambda., where .rho..sub.0 is the radius of the Al electrode 14 and .lambda. is the wavelength of the acoustic wave to be used. In this case, the diameter of the acoustic wave becomes substantially equal to the diameter of the transducer at the Fresnel focal distance. As stated above, in the known acoustic lens, the diameter of the transducer is made substantially equal to the aperture of the spherical lens portion 15 and the length of the medium 11 is made substantially equal to the Fresnel focal distance, so that uniform intensity distribution of acoustic energy can be obtained at the lens portion 15. This is the basic design principle of the known acoustic lens. This principle has been equally applied to known acoustic lenses described in references (2) to (5) and (7).
In reference (6) there is disclosed an acoustic lens in which the length of the ultrasonic wave propagating rod is set to the inverse of an odd number, particularly one third (1/3) of the Fresnel focal distance and the aperture diameter of the lens portion is set also to the inverse of an odd number, particularly one third (1/3) of the diameter of the transducer. This known acoustic lens has been developed in order to solve the following problem. In order to reduce the dumping of the acoustic energy in the water inserted between the lens and specimen, it is advantageous to shorten the working distance. Then, the radius of the lens portion and the aperture diameter have to be reduced, so that the radius of the transducer becomes shorter accordingly. However, an acoustic lens having such a small transducer and lens portion cannot be practically manufactured or can be manufactured only with difficulty. In the acoustic lens shown in the reference (6), the above-mentioned problem is solved by increasing the dimension of the transducer. However, it should be noted that in this known acoustic lens, the previously mentioned principle that the amplitude of the acoustic energy becomes uniform at the lens portion has been equally applied.
As explained above, upon designing the acoustic lens it is preliminarily noted that the simplest or uniform distribution of the acoustic energy can be attained at the lens portion and that the acoustical field at other portions has been neglected. Particularly, the known acoustic lenses have been designed without taking into account the phase of the acoustical field. Therefore, it is practically impossible to design various acoustic lenses which can be advantageously used in various applications and satisfy various requirements. In practice, almost all acoustic lenses have been manufactured in such a manner that the aperture diameter of the lens portion is made substantially equal to the diameter of the transducer and the length of the ultrasonic wave propagating solid state medium is made substantially equal to the Fresnel focal distance. That is to say, the known acoustic lenses have been manufactured by determining various parameters such as frequency, aperture diameter and aperture angle in accordance with the above mentioned design principle and the lenses thus manufactured were set to actual acoustic microscopes to check whether or not the required conditions would be satisfied. In general, the known acoustic lenses manufactured in the manner explained above were not satisfactory. Then new acoustic lenses had to be manufactured again by changing one or more parameters. In this manner, the known acoustic lenses were manufactured by a trial and error method. It is apparent that such a process is quite cumbersome and requires a very long time, and sometimes desired acoustic lenses could not be obtained. Particularly, in the acoustic lens for obtaining the V(Z) curve the phase of the acoustical field is very important, and not only does the acoustic wave have to be in-phase at the spherical lens portion, but also the amplitude of the acoustic energy has to be sufficiently large at the spherical lens portion. However, it is practically difficult to obtain an acoustic lens satisfying such conditions. This is mainly due to the fact that, according to the known design principle, the lens aperture has to be small for making the acoustic wave in-phase at the lens aperture, and therefore the amplitude or power of the acoustic wave becomes weak. However, no study has been done for finding the maximum permissible phase difference.